The present invention relates generally to manufacturing and, more particularly, to a method and apparatus for scheduling pilot lots.
The technology explosion in the manufacturing industry has resulted in many new and innovative manufacturing processes. Today's manufacturing processes, particularly semiconductor manufacturing processes, call for a large number of important steps. These process steps are usually vital, and therefore, require a number of inputs that are generally fine-tuned to maintain proper manufacturing control.
The manufacture of semiconductor devices requires a number of discrete process steps to create a packaged semiconductor device from raw semiconductor material. The various processes, from the initial growth of the semiconductor material, the slicing of the semiconductor crystal into individual wafers, the fabrication stages (e.g., etching, doping, ion implanting, or the like), to the packaging and final testing of the completed device, are so different from one another and specialized that the processes may be performed in different manufacturing locations that contain different control schemes.
Generally, a set of processing steps is performed across a group of semiconductor wafers, sometimes referred to as a lot. For example, a process layer that may be composed of a variety of different materials may be formed across a semiconductor wafer. Thereafter, a patterned layer of photoresist may be formed across the process layer using known photolithography techniques. Typically, an etch process is then performed across the process layer using a patterned layer of photoresist as a mask. This etching process results in the formation of various features or objects in the process layer. Such features may be used as, for example, a gate electrode structure for transistors. Many times, trench isolation structures are also formed across the substrate of the semiconductor wafer to isolate electrical areas across a semiconductor wafer. One example of an isolation structure that can be used is a shallow trench isolation (STI) structure.
The manufacturing tools within a semiconductor manufacturing facility typically communicate with a manufacturing framework or a network of processing modules. Each manufacturing tool is generally connected to an equipment interface. The equipment interface is connected to a machine interface to which a manufacturing network is connected, thereby facilitating communications between the manufacturing tool and the manufacturing framework. In some cases, the machine interface can generally be part of an advanced process control (APC) system. The APC system initiates a control script, which can be a software program that automatically retrieves the data needed to execute a manufacturing process.
An APC process control strategy allows a high degree of process control, desirably on a run-to-run basis, with a moderate amount of measurement data. In this control strategy, a model of a process or of a group of interrelated processes is established and implemented in an appropriately configured process controller. The process controller also receives information which may include pre-process measurement data and/or post-process measurement data, as well as information related, for instance, to the substrate history, such as type of process or processes, the product type, the process tool or process tools in which the products are to be processed or have been processed in previous steps, the process recipe to be used, i.e., a set of required steps for the process or processes under consideration, wherein possibly fixed process parameters and variable process parameters may be contained, and the like.
From this information and the process model, the process controller determines a controller state or process state that describes the effect of the process or processes under consideration on the specific product, thereby permitting the establishment of an appropriate parameter setting of the variable parameters of the specified process recipe to be performed with the substrate under consideration, wherein tool-specific internal or “low-rank” control units (substantially) maintain the parameter values, such as flow rates, temperatures, exposure doses and the like, at the targets specified by the APC controller.
Thus, the APC controller may have a predictive behavior, whose accuracy may depend on the amount of measurement data and its timeliness with respect to the current process run. The measurement data, however, may stem from different process tools performing equivalent processes.
To address the multiple tool, multiple process, and multiple product issues arising in a complicated fabrication facility, process control activities are sometimes organized into control threads. A particular control thread refers to a series of process steps that are to be performed by a specific processing tool or by a specific set of processing tools. Distinct control threads are defined for different tool, process, reticle, and/or product combinations as deemed appropriate. Incoming metrology data is associated with a particular thread, and control actions are determined for the thread to generate the process recipe or other production related parameters.
For a particular control thread to be effective in controlling a tool or process, the metrology data provided to the thread should be recent and regular. The age of the metrology data for the thread directly affects the quality of the process control. In some cases, a control thread may become obsolete if it has not received metrology data for a predetermined period of time (e.g., 2-3 days).
Prior to processing selected lots, the control system determines if the associated control thread is active. If the control thread is obsolete, a few wafers will be split out from the parent lot to form a pilot lot that will be processed and qualified prior to running the other lots associated with the obsolete thread through production. After piloting and validation, the piloting wafers may be merged back with the parent lot and continue the remaining process. They may also be kept as engineering wafers and permanently split from the parent lot for future engineering purposes. All other lots associated with the obsolete control thread are held until the thread is re-validated after measurement results from metrology operations are accepted.
Meanwhile, the tool may still process lots of mature threads after applying necessary setups (or no setup if lots waiting in the queue are of the same thread as the current recipe setup on the tool). If there are no other lots associated with active threads waiting in the queue, the tool is idle. From a tool capacity perspective, pilot lots cause capacity loss due to the potential holds or setup changes. If the tool is a production bottleneck, such capacity loss is not recoverable. Even though tools may be occupied by processing other lots with active threads, capacity loss may still occur as the result of the required setup changes.
From a fabrication perspective, pilot lots have an impact on the balancing of the production line. Delaying the dispatch of a pilot lot may result in the lots associated with the obsolete thread to accumulate in front of the tool and cause the production line to become unbalanced.
Current techniques for processing pilot lots include assigning a high priority to the pilot lot to reduce the time that the thread is idle. However, such a technique can potentially disrupt the flow of other products being processed that have to compete with the pilot lot on the basis of priority. Hence, a need exists to determine the best time to dispatch pilot lots so that process control, machine capacity, and production work-in-progress (WIP) remain well balanced.
This section of this document is intended to introduce various aspects of art that may be related to various aspects of the present invention described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the present invention. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art. The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.